1. Field of the Invention
The present invention relates to apparatus and methods for processing hydrocarbons. In another aspect, the present invention relates to apparatus and methods for processing natural gas. In still another aspect, the present invention relates to apparatus and methods for processing natural gas into liquified natural gas.
2. Description of the Related Art
The production of oil is many times accompanied by the production of natural gas. At one time, it would not be unusual to flare this natural gas. More recently, regulatory, economic, and/or public relations considerations have generally dictated that this associated natural gas be disposed of in an acceptable manner, or recovered for sale or other use, such as, for example, as a fuel in the production process, or reinjected back into the formation to assist production.
Of course, where nearby processing infrastructure exists, recovery or proper disposal of this associated gas is generally not an issue. However, in some locations, especially offshore locations, nearby processing infrastructure does not exist, the regulatory and/or economic penalties with associated gas processing, disposal or reinjection may make the oil recovery project economically unfeasible.
The liquefaction of natural gas is generally accomplished by reducing the temperature of natural gas to a liquefaction temperature of about −240° F. to about −260° F. at or near atmospheric pressure. This liquefaction temperature range is typical for many natural gas streams because the boiling point of methane at atmospheric pressure is about −259° F. In order to produce, store and transport LNG, conventional processes known in the art require substantial refrigeration to reduce and maintain natural gas at its liquefaction temperature. The most common of these refrigeration processes are: (1) the cascade process; (2) the single mixed refrigerant process; and (3) the propane pre-cooled mixed refrigerant process.
A cascade process produces LNG by employing several closed-loop cooling circuits, each utilizing a single pure refrigerant and collectively configured in order of progressively lower temperatures. The first cooling circuit commonly utilizes propane or propylene as the refrigerant, the second circuit may utilize ethane or ethylene, while the third circuit generally utilizes methane as the refrigerant.
A single mixed refrigerant process produces LNG by employing a single closed-loop cooling circuit utilizing a multicomponent refrigerant consisting of components such as nitrogen, methane, ethane, propane, butanes and pentanes. The mixed refrigerant undergoes the steps of condensation, expansion and recompression to reduce the temperature of natural gas by employing a unitary collection of heat exchangers known as a “cold box.”
A propane pre-cooled mixed refrigerant process produces LNG by employing an initial series of propane-cooled heat exchangers in addition to a single closed-loop cooling circuit, which utilizes a multi-component refrigerant consisting of components such as nitrogen, methane, ethane and propane. Natural gas initially passes through one or more propane-cooled heat exchangers, proceeds to a main exchanger cooled by the multi-component refrigerant, and is thereafter expanded to produce LNG.
Unfortunately for the above processes, the construction and maintenance of such plants is expensive because of the cost of constructing, operating and maintaining one or more external, single or mixed refrigerant, closed-loop cooling circuits.
Another penalty associated with external closed-loop cooling circuits is that such circuits require the use and storage of highly explosive refrigerants that can present safety concerns. Refrigerants such as propane, ethylene and propylene are explosive, while propane and propylene, in particular, are heavier than air further complicating dispersion of these gases in the event of a leak or other equipment failure. This is of particular concern during the offshore production and transport of LNG by ocean going vessels or other floating vessels because of: (1) the large amount of refrigerants that must be stored in order to maintain the liquefaction temperature of natural gas; and (2) the close proximity of these refrigerants to the ships crew.
A number of patents address the processing of natural gas into liquified natural gas.
U.S. Pat. No. 3,360,944 to Knapp et al. produces LNG by separating a natural gas feed stream into a major stream and a minor stream, cooling the major and minor streams to produce a liquid component, and thereafter using a substantial portion a the liquid component as a refrigerant for the process. The liquid component is vaporized while undergoing heat exchange, compressed and discharged from the process. The Knapp process results in only a minor portion of the natural gas feed stream processed into LNG.
U.S. Pat. No. 3,616,652 to Engal discloses a process for producing LNG in a single stage by compressing a natural gas feed stream, cooling the compressed natural gas feed stream to produce a liquefied stream, dramatically expanding the liquefied stream to an intermediate-pressure liquid, and then flashing and separating the intermediate-pressure liquid in a single separation step to produce LNG and a low-pressure flash gas. The low-pressure flash gas is recirculated, substantially compressed and reintroduced into the intermediate pressure liquid. While the Engal process produces LNG without the use of external refrigerants, the process inefficiently utilizes its limited refrigeration capacity upon the entire process stream without conjunctive use of multiple separation steps to offset this severe cooling requirement. Furthermore, the Engal process inefficiently expands its process stream pressure to a level that results in a substantial and highly inefficient recompression of its flash gas. Consequently, the Engal process yields a small volume of LNG compared to the amount of work required for its production, thus reducing the cost viability of the process.
U.S. Pat. No. 5,755,114 issued to Foglietta, discloses a hybrid liquefaction cycle for the production of LNG. The Foglietta process passes a pressurized natural gas feed stream into heat exchange contact with a closed-loop propane or propylene refrigeration cycle prior to directing the natural gas feed stream through a turboexpander cycle to provide auxiliary refrigeration. The Foglietta process can be implemented with only one closed-loop refrigeration cycle, as opposed to cascade type mixed refrigerant systems currently used to produce atmospheric LNG. However, the Foglietta process still requires at least one closed-loop refrigeration cycle comprising propane or propylene, both of which are explosive, not easily dispersed and must be stored on the vessels that transport the Foglietta product.
U.S. Pat. No. 6,023,942 to Thomas et al. discloses a process for producing a methane-rich liquid product having a temperature above about −112° C. (−170° F.) at a pressure that is sufficient for the liquid product to be at or below its bubble point. The resulting product is a pressurized liquid natural gas (“PLNG”), which has a pressure substantially above atmospheric pressure. While the Thomas et al. process can be implemented without external refrigeration, the product is pressurized requiring the use of specially designed heavy, thick-walled containers and transports (e.g., a PLNG ship, truck or railcar). This higher pressure, heavier walled equipment adds substantial weight and expense to any commercial project. The PLNG consumer will also require additional liquefaction, transport, and storage equipment to consume the PLNG, adding further cost to the supply and demand value chain.
U.S. Pat. No. 6,564,578, issued May 20, 2003 to Fischer-Calderon, is directed to a process for producing LNG by directing a feed stream comprising natural gas to a cooling stage that (a) cools the feed stream in at least one cooling step producing a cooled feed stream, (b) expands the cooled feed stream in at least one expansion step by reducing the pressure of the cooled feed stream producing a refrigerated vapor component and a liquid component, and (c) separates at least a portion of the refrigerated vapor component from the liquid component wherein at least a portion of the cooling for the process is derived from at least a portion of the refrigerated vapor component; and repeating steps (a) through (c) one or more times until at least substantial portion of the feed stream in the first cooling stage is processed into LNG wherein the feed stream in step (a) comprises at least a portion of the liquid component produced from a previous cooling stage.
All of the patents cited in this specification, are herein incorporated by reference.
However, in spite of the above advancements, there still exists a need in the art for apparatus and methods for processing natural gas.
This and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.